Vestibular Schwannoma (Acoustic Neuroma)

Vestibular schwannoma (VS), also called acoustic neuroma, is a benign neoplasm arising from Schwann cells (myelin-forming glial cells) of the vestibular division of the eighth cranial nerve (vestibulocochlear), at the transition zone between central oligodendroglia and peripheral Schwann cells (Obersteiner-Redlich zone). Annual incidence approximately 1-2 per 100,000, accounting for 6-8% of intracranial tumors and 80-90% of cerebellopontine angle (CPA) tumors. Typically slow-growing (mean 2 mm/year, ranging from no growth to 10+ mm/year), with about half showing no growth on serial imaging. Usually unilateral and sporadic; bilateral tumors are pathognomonic for neurofibromatosis type 2 (NF2 — autosomal dominant disorder due to NF2 gene mutation on chromosome 22q12 encoding merlin/schwannomin).

Pathophysiology and clinical features: tumor arises in internal auditory canal or extends to CPA, causes symptoms by direct nerve compression (vestibular and cochlear functions), brainstem compression with larger tumors, and adjacent cranial nerve involvement (trigeminal, facial). Koos classification of size: T1 (intracanalicular), T2 (small CPA <1 cm), T3 (medium CPA 1-3 cm), T4 (large CPA >3 cm with brainstem compression). Hannover classification similar. Clinical presentation: unilateral progressive sensorineural hearing loss (95% — most common, often gradual but may be sudden), tinnitus (63%), balance disturbance/dizziness (61%), facial numbness/trigeminal involvement (with larger tumors), facial nerve weakness (rare with small tumors despite proximity), headache, ataxia, brainstem symptoms (cranial neuropathies, hydrocephalus, long tract signs with very large tumors). Asymmetric high-frequency hearing loss in audiometry is the key initial finding.

Diagnosis is by audiometry showing asymmetric sensorineural hearing loss particularly at high frequencies, gadolinium-enhanced MRI of the internal auditory canal/CPA (gold standard — high-resolution thin-section sequences with FIESTA/CISS for screening, T1 post-contrast for diagnosis showing enhancing mass with characteristic ice cream cone appearance), CT for bony erosion if MRI contraindicated, vestibular testing (VNG, VEMP, caloric testing) showing decreased vestibular response, brainstem auditory evoked responses (BAER) showing prolonged interpeak latencies, genetic testing for NF2 if bilateral or family history. Management options: observation with serial MRI (especially small tumors, elderly patients, those with serviceable hearing wanting preservation) — annual or biannual MRI; stereotactic radiosurgery (SRS — gamma knife, cyberknife, linear accelerator) — single session 12-13 Gy or fractionated, achieves >90% tumor control with good hearing preservation rates (50-75% short-term), facial nerve preservation 95%; microsurgical resection — three approaches: translabyrinthine (sacrifices hearing, best facial nerve preservation, good for non-serviceable hearing), retrosigmoid (preserves potential hearing, larger tumors, posterior fossa exposure), middle fossa (best for small intracanalicular tumors with serviceable hearing, complex). Choice depends on tumor size, hearing status, age, comorbidities, surgeon expertise. Postoperative monitoring for hearing, facial nerve function, CSF leak, meningitis, hydrocephalus. NF2 management requires multidisciplinary team with hearing rehabilitation, intervention timing, bevacizumab for non-resectable tumors, brain stem implants or auditory brainstem implant for bilateral hearing loss.